Directional coupler having interdigital comb electrodes

ABSTRACT

A directional coupler comprising a dielectric substrate having two opposing faces carrying a sheet electrode on one face and two comb like electrode structures on the other face. Each comb electrode has finger like electrodes projecting from a bus bar towards the other comb. Varying the length of finger electrodes and the amount of overlap varies the amount of signal power coupled between the two combs. Couplers may be arranged to couple up to 100% of signal power. When half signal power is coupled, a 3dB coupler, various microstrip circuits may be built e.g. balanced mixers, power dividers, or power combiners.

This invention relates to directional couplers formed in microstrip or stripline. Such construction involves use of thin strip electrodes on the flat faces of a dielectric substrate.

One microstrip arrangement comprises a dielectric flat substrate with a ground plane electrode covering one surface and with a narrow strip electrode on the other face. An electromagnetic wave signal applied between the ground and the strip electrode is transmitted along the strip.

One known form of strip line directional coupler comprises two strips of a conductor arranged parallel to one another on the same surface of an insulating substrate. Provided the conductor spacing and length of the coupled system is correct power will be transferred from one strip to the other over a particular frequency range. Such a coupler is termed an edge coupled microstrip directional coupler.

A disadvantage with this edge coupled microstrip coupler is the accuracy required in spacing the parallel electrodes and in dimensioning the strips to ensure the desired amount of coupling.

According to this invention a directional coupler comprises a dielectric substrate having two opposing faces, a ground sheet electrode on one face of the substrate, and two spaced, electrically isolated comb electrodes on the other face of the substrate arranged so that a portion of a signal power may be capacitively coupled from one comb to the other.

The invention will now be described by way of example only with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of one form of the invention;

FIGS. 2, 3, 4, 5 are graphs showing power transfer in the coupler; and

FIG. 6 is a diagrammatical view of a balanced mixer.

The directional coupler shown in FIG. 1 comprises a flat substrate 1 of polyolefin material e.g. Polyguide (Trade Mark) covered on one face with a ground or earth sheet copper electrode 2 and carrying on its opposite face two interdigital comb-like copper electrode structures 3, 4. These structures 3, 4, each comprise a bus bar 5, 6 with fingers or stubs 7, 8 projecting normal thereto. It can be seen that the fingers 7, 8 in the two combs 3, 4 are interdigital and equispaced. Lead strips 9, 10, 11, 12, connect the combs to ports P₁ P₂ P₃ P₄, so that an input (or output) signal may be applied between strip 9 and the ground plane at port P₁, etc.

The two comb structures can be considered to be a four port coupled transmission line system having two modes of propagation. The behaviour of this transmission line is dependent on the dimensions (and impedance) of the two combs 3, 4. Let the length of the interdigital combs 3, 4 be L, the length of fingers in one comb 3 be 1₁ and in the other comb 4 be 1₂, the finger overlap be t, the spacing of adjacent fingers be s, and the fingers on each comb be spaced p apart.

If 1₁ = 1₂, i.e. the combs are symmetric, and a constant frequency signal is applied to input port P₁, then the output at ports P₃ and P₄ for various values of comb length will be as shown in FIG. 2, neglecting losses due to radiation resistance, reflection and dielectric absorption. At a point marked L_(c) all power has been transferred from comb 3 to comb 4, i.e. there is zero output at P₄ and all power is at P₃.

For a given length L, varying the input signal frequency results in varying amounts of power being transferred to output ports P₃, P₄ as shown in FIG. 3. Maximum transfer from input port P₁ to output port P₃ occurs at a center frequency f_(c).

The theoretical basis for this (when 1₁ = 1₂) is as follows. A general signal applied to input port P₁ only can be analyzed in terms of the symmetric and anti-symmetric modes of propagation. At any point along the combs 3, 4 there will be a phase difference between the modes on account of their different velocities, and the energy distribution between the combs 3, 4 at that point is given by (1- cos φ)/(1+ cos φ) where φ is the phase angle between the two modes. Thus there is a periodic transfer of energy from one comb to the other comb, with maximum transfer occurring when φ= = π. This transfer is periodic in length at a given frequency (FIG. 2) and, for a dispersionless system, periodic in frequency for a given length L (FIG. 3).

For the case where 1₁ ≠≠ 1₂ then the modes become unbalanced and the maximum power transferred is less, (FIG. 4). By choice of L, 1₁ and 1₂ the amount of power transfer can be made equal to 1/2 i.e. a 3dB transfer in which case the two curves of FIG. 4 just touch as shown in FIG. 5. Thus power applied to input port P₁ is split equally between output ports P₃ and P₄ with zero signal (in theory) at port P₂.

For a given length L and frequency f_(c) the amount of coupling between combs varies with finger overlap t.

As already noted behaviour of the coupled transmission line is dependent on the physical characteristic of the two combs 3, 4. The frequency of operation is governed by the various stop bands arising from the periodic impedance mismatches and resonance effects due to the spacing s, p and length of the fingers 7, 8.

Thus for example a coupler can be constructed so that the first resonance band occurs when 1₁ = λ/4 i.e. a lower frequency than that of the stop band which occurs when the distance P between adjacent fingers on one comb =λ /2, λbeing wavelength.

A broad band coupling has been found in a coupler of this invention when the frequency of operation f_(o) is about half that frequency at which λ/4 resonance occurs in the length of the fingers i.e. when 1₁ is approximately λ_(c) /8 where λ_(c) is the wavelength at the center frequency f_(c). In such an example a flat bandwidth of about f_(c) ± 20% may be achieved.

As an example a broadband 3dB coupler has been constructed to operate over the frequency range 5.3 to 6.6 GH_(z) and had the following dimensions L= 32mm, 1₁ = 5mm, 1₂ = 4mm, P== 4 mm, s== 1 mm, finger overlap t= 2mm, width of bus bars= 1mm, thickness of electrodes= 0.035mm, substrate= 1.6mm thick Polyguide (Trade Mark).

In another variation the electrode assembly of FIG. 1 may be covered with another sheet of dielectric, itself backed with a ground sheet electrode. This arrangement is useful for reducing radiation losses, especially with low dielectric constant substrates, and constitutes a triplate or stripline configuration. For this case the exact dimensions of the comb electrodes may differ from those for the open, or microstrip, variation for the same coupling value.

As another example a 10 GHz 3dB coupler has been built in a triplate configuration and had the following dimensions: L= 28mm, 1₁ = 2.3mm, 1₂ = 0.9mm, p = 2.4mm, s= 0.6mm, width of bus bar 5= 0.8mm, width of bus bar 6= 1.0mm, thickness of dielectric 1= 1/16 inch and dielectric constant= 2.4.

The various parameters to be considered in designing the coupler are therefore:

ratio 1₁ /1₂ - which sets maximum power transfer between combs

1₁ - sets the first cut off frequency

t, s - the coupling coefficient depends on t, s; the coupling coefficient in turn affects the value of f_(c) for a given L

L - sets the device center frequency

width of bus bars affects the input impedance and hence the matching conditions.

The finger lengths 1₁, 1₂ may be constant, as shown, or may vary along the coupler length. Also the finger separation p, s may be constant or varying. The finger overlap t may be positive as shown, zero, or the combs may be further still apart in which case there will be a gap between the two combs i.e. a negative value of t. The two combs of a coupler may be spaced parallel apart or at a varying distance apart.

As shown in FIG. 1 a coupler comprises a substrate with four ports P₁, P₂, P₃, P₄ to which other devices (e.g. coaxial lines) may be attached. In practice the coupler shown may be a part of a much larger strip line circuit. Matching in to the coupler from the input and output lines is by tapered sections of transmission line.

For a coupler having symmetric combs 1₁ = 1₂, for an input to port P₁ the outputs at P₃, P₄ will be 90° out of phase, for any comb length not just L_(c). However for non-symmetric combs, 1₁ ≠ 1₂, the phase between outputs P₃, P₄ is not 90° . If a 90° phase is required, one of the lead electrodes e.g. at P₃ may include a section of comb line whose dispersive characteristics are such that the phase variation between P₃ and P₄ is reduced to that desired.

In a 3dB coupler signals at the output ports P₃, P₄ are either out of phase or in phase depending on the input port used. When a signal is applied to input port P₁ only, the phase difference between signals at P₃ and P₄ is 180°. When a nonsymmetric comb structure is employed and a signal is applied to input port P₂ only, the signals at output ports P₃, P₄ are in phase. Such a phase relation occurs at the center frequency f_(c) ; at frequencies away from f_(c) this phase relationship varies linearly with variation from f_(c).

Uses of such a 3dB coupler include in-phase signal power dividers, i.e. signal input to P₂ ; out of phase signal power dividers i.e. signal input to P₁ ; power combiner i.e. two equal in-phase signals one applied to P₁ the other to P₂ and power out of P₃.

Another use of a 3dB coupler is shown diagrammatically in FIG. 6 having identical reference characters as FIG. 1. Additionally it comprises two microwave diodes D₁, D₂ connected to the outputs P₃ P₄ and to a common IF output and forms a balanced mixer. A d.c. bias is applied to the diodes from a voltage source not shown. A signal from a local oscillator is applied to input port P₁ and a modulated signal applied to input port P₂.

In operation the coupler splits the local oscillator power out of phase and the received power in phase. The signals are mixed by the diodes to provide an IF signal. The local oscillator noise cancels on recombination at the diode outputs. 

I claim:
 1. A directional coupler comprising a dielectric substrate having two opposing faces, a ground sheet electrode on one face of said substrate, first and second elongated bus bars disposed in spaced relation to one another on the other face of said substrate, a first series of finger electrodes extending from said first bus bar at spaced locations distributed along substantially the entire length of said first bus bar, said first series of finger electrodes being disposed transverse to the direction of elongation of said first bus bar and projecting therefrom toward said second bus bar, a second series of finger electrodes extending from said second bus bar at spaced locations distributed along substantially the entire length of said second bus bar, said second series of finger electrodes being disposed transverse to the direction of elongation of said second bus bar and projecting therefrom toward said first bus bar, said first bus bar and its respective first series of finger electrodes being separated from said second bus bar and its respective second finger electrodes to form first and second electrically isolated interdigital comb structures which are capacitively coupled to one another, means providing an input port and an output port for each of said comb structures, said input and output ports being disposed at opposite ends of each of said bus bars respectively for applying signals to and extracting signals from the coupler, the two output ports for said two comb structures being located at adjacent ends of said first and second bus bars respectively, whereby a proportion of signal power applied to the input port at one end of one of said comb structures is coupled to the remote output port at the other end of the other comb structure.
 2. A coupler according to claim 1 wherein the finger electrodes in each comb structure are of uniform length.
 3. A coupler according to claim 1 wherein the finger electrodes on one comb structure are longer than the finger electrodes on the other comb structure.
 4. A coupler according to claim 1 wherein the coupler is arranged to couple about one half the signal power from one comb structure to the other.
 5. A coupler according to claim 1 and comprising a further substrate having two opposing faces, one carryng a further sheet electrode and the other being in contact with said two comb structures.
 6. A coupler according to claim 1 wherein the two spaced bus bars are parallel to one another. 